PT100 to Siemens LOGO! via 4-20mA Transmitter Wiring Guide

Overview

This guide explains how to connect a three-wire PT100 RTD (Resistance Temperature Detector) to a Siemens LOGO! programmable relay through an industrial 4-20 mA temperature transmitter. The solution uses a 500 Ω precision sense resistor on the LOGO! AM2 analog input module to convert the 4-20 mA current loop into a 0-10 V signal that the AM2 module can read natively.

The source hardware described in the original engineering question is a LOGO! 12/24 RCE with order number 6ED1 052-1MD00-0BA7 (LOGIC module) paired with an AM2 analog input expansion 6ED1 055-1MM00-0BA1. The transmitter is a 24 V DC loop-powered PT100-to-4-20 mA device with a -50 to +150 °C span. The same wiring principle applies to the newer LOGO! 0BA8 generation (6ED1 052-1MD08-0BA8 / AM2 6ED1 055-1MM08-0BA1) and the LOGO! 8.3 (0BA8 FS:05) firmware line.

Hardware Identification

Verify each catalog number on the device label before commissioning. Mixing an AM2 (0-10 V / 0-20 mA) with an AM2 RTD module (6ED1 055-1MD00-0BA1 / 6ED1 055-1MD08-0BA1) is a common substitution error: the RTD variant accepts PT100/PT1000 directly and does not require an external transmitter.

Selecting a 4-20 mA PT100 Transmitter

Choose a head-mount or DIN-rail transmitter that accepts a three-wire PT100 input and outputs a current loop proportional to temperature. The transmitter must be loop-powered (drawing its operating current from the 4-20 mA loop itself) to keep wiring to two wires only at the AM2 end.

Confirm the following before purchase:

• Output range is 4-20 mA, not 0-20 mA or 0-10 V.
• Configured span matches the process: here -50 to +150 °C.
• Sensor wiring mode is 3-wire (matches the PT100 lead configuration).
• Supply voltage is 24 V DC, polarity-protected.
• Accuracy ≤0.3 °C or better; output load capability ≥500 Ω at 24 V.

Calculating the 500 Ω Sense Resistor

The AM2 analog input module on the LOGO! measures 0-10 V or 0-20 mA depending on the parameter block assignment in LOGO! Soft Comfort. The 4-20 mA current loop is converted into a 0-10 V voltage signal by inserting a precision resistor in series with the loop return. Apply Ohm's law to size the resistor:

V = I × R ⇒ R = V / I

To map 20 mA to 10 V at full scale:

R = 10 V / 0.020 A = 500 Ω

The resulting voltage span across the resistor is:

Resistor specification checklist:

• Resistance: 500.0 Ω nominal.
• Tolerance: ≤0.1 % to keep scale error below ±0.05 % of span.
• Temperature coefficient: ≤15 ppm/°C (Caddock, Vishay, Yageo MELF).
• Power rating: P = I² × R = 0.020² × 500 = 0.20 W; choose a 0.5 W part for headroom.
• Voltage coefficient: Avoid carbon composition; use metal film or wirewound.

PT100 Three-Wire Wiring

PT100 sensors come in 2-wire, 3-wire, and 4-wire constructions. The 3-wire version is the most common industrial form because it cancels the lead-wire resistance in a Wheatstone bridge inside the transmitter. The transmitter must be configured for 3-wire mode; the sensor leads connect to terminals labeled typically as 1, 2, and 3 or A, B, B (two wires share a common terminal internally).

Complete Loop Wiring

The current loop is closed through the AM2 input. The 24 V DC supply, the transmitter, the 500 Ω resistor, and the AM2 input share one series circuit. Polarity must be observed at the transmitter and the AM2.

Wiring procedure:

1. Connect the PT100 leads (terminals 1, 2, 3) to the transmitter's RTD input. Tighten to the torque specified on the transmitter datasheet (typically 0.5-0.6 Nm for Phoenix Contact MINI Analog).
2. Connect the transmitter's + terminal to the LOGO! 24 V DC supply (terminal L+ on the base module).
3. Connect the transmitter's - terminal to one end of the 500 Ω resistor.
4. Connect the other end of the 500 Ω resistor to the AM2 input terminal I1 (or I2).
5. Bridge AM2 terminal M (analog ground) back to the LOGO! 24 V DC common M.
6. Verify polarity with a multimeter: red lead on PSU +, black lead on AM2 I1 should read +24 V DC (no load) and roughly +14 V DC under loop conditions.

Configuring the AM2 Module

On the LOGO! AM2 module, the analog input type is set in LOGO! Soft Comfort, not with DIP switches on the 0BA7/0BA8 hardware. Open the program, navigate to Tools → Parameter VM Mapping or directly into the AI block, and select:

• AI1 type: 0-10 V (voltage mode).
• AI2 type: 0-10 V or unused, depending on the application.
• Resolution: 10-bit (default). One LSB equals 10 V / 1024 = 9.77 mV.

The raw count from the AM2 in 0-10 V mode ranges from 0 to 1000 in the LOGO! variable memory:

• 2.000 V (4 mA, -50 °C) → raw = 200.
• 10.000 V (20 mA, +150 °C) → raw = 1000.

Scaling in LOGO! Soft Comfort

Use the Analog Amplifier block (function block B001 in the Special Functions library) to convert the raw AI value into engineering units (°C). Two common approaches are detailed below.

Method A - Linear scaling with gain and offset

The transfer function from raw count to °C is:

T(°C) = (raw - 200) × (200 / 800) - 50

where 200 is the count at -50 °C, 800 is the span in counts (1000 - 200), and 200/800 = 0.25 is the gain. The Analog Amplifier block supports one gain and one offset:

• Gain: 0.25
• Offset: -100 (engineering units; represents the value to subtract after the gain stage: -50 °C minus 0.25 × 200)

The block output is the engineering value to be displayed on the LOGO! built-in TD/text panel or passed to a Threshold Trigger.

Method B - Two-stage math using the Analog Math block

If your firmware supports the Analog Math block (0BA8 FS:02 and later), apply the explicit equation:

AI_raw := VW0          // raw count from AM2 AI1
delta  := AI_raw - 200 // 0 = -50°C, 800 = +150°C
T_C    := delta * 0.25 - 50

Add a hysteresis stage before any control output to prevent chattering around the setpoint (recommended ±1 °C hysteresis for heater-control applications).

Threshold trigger example

To trigger a relay Q1 when T_C rises above 80 °C:

• Threshold Trigger block (B007):
• On threshold: 80 (°C)
• Off threshold: 75 (°C)
• Hysteresis span: 5 °C
• Sensor: connect block input to the Analog Amplifier output from Method A.

Verification and Calibration

1. Power up the LOGO! and check the AM2 LED: green = OK, red = wiring or overload fault.
2. Use a calibrated mA source (e.g., Beamex MC6 or WIKA Pascal) in place of the transmitter to inject known currents. Verify 4 mA → raw 200, 12 mA → raw 600, 20 mA → raw 1000.
3. Apply a 0 °C ice-bath reference to the PT100 probe. Reading should be 0 °C ±0.5 °C after one-point trim.
4. Apply a 100 °C boiling-water reference (compensate for altitude; subtract ~1 °C per 285 m of elevation above sea level).
5. Check loop voltage at the transmitter terminals with the loop live. The voltage should stay within the transmitter's compliance range, typically 8 to 28 V DC. Below 8 V DC the loop sags and linearity suffers.
6. Record the cold-junction drift over 24 h; with a precision 500 Ω resistor the drift should be <0.1 °C / 10 K ambient change.

Alternative - Direct PT100 with AM2 RTD Module

If transmitter cost, accuracy budget, or cabinet space are concerns, consider the AM2 RTD expansion module:

The AM2 RTD connects a 2-wire or 3-wire PT100 directly. No external transmitter, no 500 Ω resistor, and the LOGO! Soft Comfort "Sensor type" parameter is set to PT100. The AI raw value already represents °C, so the scaling block is omitted.

The trade-off is accuracy: the AM2 RTD module uses internal reference resistors and typically achieves ±1.0 °C (Class B equivalent) versus ±0.3 °C of a high-quality external transmitter. For HVAC, building automation, or general machinery monitoring, the AM2 RTD is usually sufficient. For process control with audit-grade accuracy, the 4-20 mA transmitter path described in this article is the better choice.

Common Pitfalls and Diagnostics

FAQ